Capacitive Position Sensors Capacitive position sensors (capacitance sensors) provide position resolutions to 0.01 nanometer. Ideal for nanometrology applications ... - Physik Instrumente

Catalogue Capacitive Position Sensors Capacitive position sensors (capacitance sensors) provide position resolutions to 0.01 nanometer. Ideal for nanometrology applications ...

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Glossary

Measurement Range

The measurement range depends on the size of the active sensor area as well as on the electronics used. Due to PI's proprietary signal conditioner electronics design, the mid-range distance is always identical to the selected measurement range. The probe-to-target gap may vary from

50% to 150% of the measurement range (see Fig. 14). The sensor capacitance is the same as that of the reference capacitance in the electronics. Different reference capacitances can be used to extend the nominal (standard) measurement range (see Fig. 15).

Fig. 14: Definitions: measurement range and mid-range distance have identical values

Target

Two-electrode capacitive sensors consist of two electrodes, named probe and target. Single-electrode sensors measure against a surface that is called the target. The target surface is, in principle, a conductive material electrically connected to ground. Measurement against semi-conductors is possible as well.

While two-plate capacitive sensors consist of two well-defined

high-quality planes, with single-plate sensors, target surface characteristics can influence the results. A curved or rough surface will deteriorate the resolution because the results refer to an average gap (see Fig. 16 and 17). Surface shape also influences the homogeneity of the electric field and thereby the measurement linearity. For factory calibration, a target plane that is considerably larger than the sensor area is used.

Fig. 15: Measuring ranges of different PI capacitive position sensors (standard ranges in blue, extended ranges in black)

Environment

Precision measurement with nanometer accuracy requires minimizing environmental influences. Constancy of temperature and humidity during the measurement are as essential as cleanliness.

Electronics from PI are basically very temperature stable. Temperature drift is under 0.2% of full measurement range with a change of temperature of 10 C°. Temperature changes also cause all material in the system to expand or contract, thus

changing the actual measured gap.

The influence of a change in relative humidity of 30 percentage points is less than 0.5% of the measurement range. Condensation must always be avoided. Dusty or damaged sensor surfaces will also worsen the measurement quality. Environmental conditions at the time of calibration are noted on the calibration sheet PI provides with each individual system.

Fig. 16: Roughness of the target surface downgrades resolution and linearity

Fig. 17: Curved surfaces lead to an averaged distance measurement